To control the power supply to a load with defined timing, the load current through the load is usually, in the case of so-called phase angle control, switched on with the aid of thyristors or triacs only at a certain phase angle after a zero crossing of the main AC voltage by a control pulse, and switches off again by itself at the next zero crossing. A drawback of the phase angle the control is the low power factor, since even with ohmic .load a certain reactive power component--and a high one case of large phase angles--is always present. In addition, there is often a holding current problem in phase angle control, since inadvertent switch-off of the semiconductor switch--particularly at low load crrents--has to be prevented. Finally, a high degree of interference suppression is necessary in phase angle control on account of the sudden voltage decrease occurring at load current switch-on time, and of the HF problems this entails.
Unlike in phase angle control, the power supply to the load in reverse phase angle control is controlled with the aid of semiconductor switches, that can be fully switched off, and are preferably field-effect transistors, such that the load current is switched on in the zero crossing of the main AC voltage and switched off again at a certain phase angle (when the current set is reached). Since this minimizes the reactive power component, the active power component is at a maximum and an optimum efficiency is thereby obtained; since no sudden voltage decrease occurs when the load current is switched on and the switch-off edge of the curve can have a controlled course, no interference suppression measures are needed.